Mysterious emissions of radio light from the far reaches of the universe are the next big thing in modern radio astronomy. Fleeting flurries of radio waves, called fast radio bursts (FRBs) reach earth from faraway galaxies, emitting as much energy in a millisecond as the sun does over weeks.
In spite of being the brightest radio bursts found in nature, however, these will o’ the wisps of the cosmos are so transient that astrophysicists have only been able to ‘see’ them momentarily using large radio telescopes. Ever since the first FRB was picked up by radio astronomers more than 15 years ago, they have identified hundreds, and the list is getting longer by the day.
What do we know about FRBs?
We know almost nothing about the precise origins of FRBs and why they appear in such short, sharp bursts – other than that these celestial electromagnetic impulses probably come from the embers of dying stars. Some FRBs are ‘one-off’ phenomena: spotted just once and never detected again; others are repeaters, flashing earth intermittently like some ghostly lighthouse in the depths of space.
An international team of astronomers has now published the results of its exhaustive study on a repeating FRB from a distant galaxy that offers new clues about the origins of these mysterious radio flashes. The report was published in the journal Science on May 12.
The astronomers tried to figure out what produces an FRB by studying its local environment to determine the sources that could exist in, or create, such environments. They targeted a repeating FRB, called FRB 20190520B (they are christened by the date of their discovery, in this case: May 20, 2019), using the Green Bank Telescope in the U.S. and the Parkes Observatory in Australia, and recorded hundreds of bursts from it.
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A view of the Green Bank Telescope. | Photo Credit: greenbankobservatory.org, CC BY 3.0
What did the astronomers find?
They discovered that the FRB’s Faraday rotation measure – an indicator of its magnetic field strength – was highly variable and that it reversed direction twice. This magnetic reversal, they believe, has to do with the FRB source orbiting a binary star system where the companion star is probably a massive star or a black hole.
“We have used magnetic fields as probes to study the FRB’s local environment,” Reshma Anna-Thomas, lead author of the study, said in an email to The Hindu. “We saw that the magnetic field in our sightline flipped in a few months, which is tiny on astronomical timescales. The value of the magnetic field and electron density was also found to vary around this source, which indicates a very turbulent magnetised plasma environment.”
Using these observed features, the researchers modelled the variations as being the result of a wind from a massive binary companion star. The wind of a star is a rapid stream of ejected material. In other words, the magnetic reversal likely happened when the radio signals passed through a turbulent, magnetised screen of plasma in the binary stellar system.
What do the findings mean?
This conclusion ties in with an older discovery of a strikingly similar binary system in the Milky Way galaxy, including the magnetic field reversal. “This FRB, [called] FRB 20190520B, is very similar to other repeating FRBs in energy scales, narrow banded emission, temporal widths etc.,” Dr. Anna-Thomas said. “But our study gives one of the most convincing pieces of evidence that this source could be in a binary system.”
Thus, she added, it is possible that “all repeating FRBs could be in binaries” but differ in their local conditions, like the orbital period or the orbital inclination. “Constant long-term monitoring of these FRBs is necessary to make a final call on this.”
Cosmologists believe that learning more about such changes in the magnetised environment around FRBs could eventually help track down their origins. To do this, astronomers have a whole new generation of radio telescopes at their disposal. They include the Very Large Array and Deep Synoptic Array-110 in the U.S., China’s Five-hundred-meter Aperture Spherical radio Telescope, the Australian Square Kilometre Array Pathfinder, India’s upgraded Giant Metre-wave Radio Telescope, Germany’s Effelsberg Radioteleskop, South Africa’s MeerKAT, and the Low-Frequency Array in the Netherlands.
Why do radio telescopes matter?
Until the early 1930s, astronomers depended on the limited visible part of the electromagnetic spectrum to make observations, unaware of the enormous potential of the radio band lying at one end of the spectrum. Their long wavelengths allow radio waves to traverse intergalactic space without interruption, making them an ideal tool to identify radio emissions from faraway heat sources.
In 1933, Bell Labs asked physicist and radio engineer Karl Jansky to find out if static was disrupting transatlantic radio communication. Jansky’s investigations led him to the accidental discovery of radio waves coming from the centre of the Milky Way galaxy. He wanted to study the signals in detail and suggested Bell Labs build a large dish antenna. But Bell Labs, merely interested in confirming that the static was not a problem for transatlantic communication, transferred Jansky to an obscure project unrelated to radio astronomy.
Fortunately, Jansky’s pioneering findings endured and inspired other scientists to develop radio astronomy, thanks to which we know about intergalactic phenomena like pulsars (fast spinning neutron stars), dark matter, the cosmic microwave background (signals left over from the universe’s birth) and, of course, FRBs.
Radio astronomers today are much better off with telescopes that can even localise FRBs with arc-second precision, so that observations in other wavelengths could hunt for the FBR’s host galaxy. So when a radio telescope spots an FRB, astronomers try to determine its dispersion value: the extent to which the FRB is stretched out when it reaches earth. From this, it is possible to calculate the distance to the FRB’s source.
By connecting dots like these, astronomers try to unravel cosmic mysteries and better understand the universe, of which hardly a fraction is known.
Prakash Chandra is a science writer.
